The pilot of any aircraft needs to know its relative velocity in relation to the air, i.e. in relation to the wind. This velocity is determined using probes for measuring the static pressure Ps and the total pressure Pt, and also sensors for measuring the angle of attack α and the angle of sideslip β. α and β provide the direction of the velocity vector in a reference system, or reference frame, associated with the aircraft and Pt-Ps provides the modulus of this velocity vector. The four aerodynamic parameters therefore enable the velocity vector of any aircraft, such as for example an aeroplane or a helicopter, to be determined.
The measurement of the total pressure Pt is usually performed using what is called a Pitot tube. This is a tube open at one of its ends and obstructed at the other. The open end of the tube substantially faces the flow.
The airstream located upstream of the tube is progressively slowed down until reaching an almost zero velocity at the inlet of the tube. The pressure of the air increases as the velocity of the air decreases. This increased pressure forms the total pressure Pt of the air flow. Inside the Pitot tube, the air pressure obtaining therein is measured.
In practice, the air flow may convey solid or liquid particles, such as for example water from clouds, which are liable to penetrate into the Pitot tube and accumulate in the tube at the obstructed end. To prevent such an accumulation from disturbing the pressure measurement, the obstructed end is generally provided with one or more drain holes and with water traps, so as to avoid any risk of obstructing the lines responsible for transmitting the total pressure to the pressure sensors located inside the fuselage of the aircraft or to the instruments on the instrument panel of the aircraft.
The drain holes serve to remove liquids and possible particles that may penetrate into the tube. These holes are particularly useful in the case of flight in a water-laden atmosphere, where the water is in the liquid state (which may be super cooled) or is in the solid state that can arise in case of icing conditions.
Flowing simultaneously through such a hole are water, particles and a portion of the air entering the Pitot tube. Thus, the air in the tube is not completely slowed down and the measurement of the total pressure Pt is thereby slightly altered. More precisely, the more it is endeavoured to prevent significant accumulation of water or particles, by increasing the size of the drain hole, the more the measurement of the total pressure is altered. Conversely, the more it is endeavoured to improve the measurement of the total pressure Pt, by reducing the size of the drain hole, the greater the risk of water or particles accumulating. With a Pitot tube, it is therefore necessary to make a compromise between the quality of the measurement of the total pressure Pt and the risk of disturbing the measurement because of the penetration of water and particles conveyed by the air flow where the measurement is carried out. It is therefore not possible for the size of the drain holes to be greatly increased in order to improve their effectiveness.
Over the operational lifetime of aircraft, the drain holes become contaminated, because of ingestion of dust, insects, residues of plant matter or other foreign bodies. Because of their size and the position of the Pitot tubes on the fuselage of an aircraft, it is not very easy for the integrity of the drain holes to be periodically checked. There is no provision to verify these holes before each flight and in-flight checking is impossible. This may have an impact on flight safety.
The drain holes of Pitot tubes are currently checked visually. The operator responsible for aircraft maintenance inspects the drain hole or holes using a small lamp. Should foreign bodies or an anomaly be observed, the probe is removed and its pneumatic circuits cleaned. This operation is all the more awkward the larger the aeroplane. Access to the probe and to the drain holes, which generally have a diameter of less than 1 mm, is difficult. Therefore, the operation is carried out only rarely. It is frequently the case that the checking periodicity is more than one year.
Of course, such time intervals are not acceptable for aeroplanes flying over countries where the atmosphere is highly polluted, when there are sandstorms or volcanic clouds, or more simply in regions where the presence of nest-building insects, such as mason bees, is high.
It frequently happens that aeroplane pilots report problems of velocity measurement fluctuation during flights that have encountered strong precipitation.
Solutions have been proposed for reducing the impact of the drawbacks associated with drain holes.
Mention may be made of pressure probes in which the operating principle makes it possible to provide a drain circuit of large cross section, as described for example in the patent published under No. FR 2 823 846 and filed on 24 Apr. 2001. This is a probe for measuring the total pressure at a fluid stagnation point. More precisely, this probe takes at least two airstreams from an air flow and brings them into contact with each other so as to slow them down. The pressure in the zone where the air is slowed down is measured and this measured pressure gives the total pressure of the flow. Such a probe makes it possible for the dimensions of the drain holes to be considerably increased. However, this solution has the drawback of significantly affecting the design and the intrinsic failure modes of the pressure probe function, and therefore of requiring a much longer time to be certified and to be installed in volume on aircraft in service.
Moreover, the measurement of the static pressure can be made using a probe flush with the skin of the aircraft and possessing a duct opening substantially perpendicular to the direction of the flow, inside which duct the pressure of the air is measured.
It may happen that certain impurities are deposited on internal walls of the duct. Over the course of time, the deposited impurities create clumps of impurities that are attached to the internal walls and may disturb the flow of the fluid in the duct by modifying its geometry. Impurity deposits and clump formation are encountered quite frequently when the fluid flow rate in the duct is not sufficient to displace deposited impurities. The deposits are caused either by impurities adhering to the internal walls of ducts or by the accumulation of impurities due to microturbulence in calm fluid flow zones.
Should the air inlet ducts of such a probe be partially obstructed, it is the response time of the probe which is affected. Should there be total obstruction, the static pressure measurement itself is false. For such a probe, a self-cleaning device has been developed for cleaning the internal walls, as described in patent No. FR 2 910 357 filed on 20 Dec. 2006. The object of this device is to warn of, and to a certain extent eliminate, the blockage phenomenon. These probes make it possible to use the principle whereby the motional impedance of the driving element is modified so as to detect a partial or complete blockage of the duct in question. However, they have the drawback of testing only the internal volumes that are equipped with the deblocking device; for example, if only the drain holes are equipped therewith, no blockage of the nose of the probe via which the flow of air penetrates the probe is detected.